How to quickly check if a zip file is corrupted?

Question

Hello,

Does anyone have any ideas for how to pragmatically quickly check if a zip file is corrupted based on file size? Ideally the best way to check if a zip is corrupted is to do a CRC check but this can take a long time especially if there is a lot of large zip files. I would be happy just to be able to do a quick file size or header check.

Thanks in advance.

Answer 1

This page says that the compressed size is 4 bytes starting from byte 18. You could try reading that and comparing it to the size to the file.

However, I think it's pretty much useless for checking if the zip file is corrupted for two reasons:

1. Some zip files contain more bytes than just the zip part. For example, self-extracting archives have an executable part yet they're still valid zip.
2. The file can be corrupted without changing its size.

So, I suggest calculating the CRC for a guaranteed method of checking for corruption.

Answer 2

DotNetZip, a free open source library for handling zip files in .NET languages, supports a CheckZip() method that does what you want. There are various levels of assurance available at your option. The basic level just checks consistency of metadata. The most complete level does a full extraction of the zip file into a bitbucket to verify that the actual compressed data is not corrupted.